In modern redundant hydraulic control systems there is frequently a requirement that any errors arising be indicated, and that if a certain number of errors occurs, a switching logic associated with the control system perform special safety switching operations, for example, deactivation of the system or return of the actuator to a neutral position.
For monitoring redundant electrohydraulic actuators, two types of hydromechanical circuits are used at the pressure ports of the servovalves (electrohydraulic servovalves and electrohydraulic amplifiers):
A. Redundant servovalve circuits. The servovalve pressure ports are arranged in such a way as to permit control of the actuator and the detection of errors by comparison of the pressures through use of a comparator. If the pressure difference between two servovalves exceeds a prescribed value, the comparator initiates a reaction in the switching and monitoring logic.
B. Switching and monitoring logic (hydrologic). Following initiation by the comparator, the pressure medium supply for servovalves is irreversibly deactivated by the hydrologic. The irreversibility of the servovalve deactivation represents an error storage. By means of a solenoid valve, the pressure medium for the servovalves can again be activated, if, at the instance of the activation command, no new deactivation is triggered by the comparator due to a possible pressure differential existing between the servovalves. An additional solenoid valve circuit permits cutoff of the pressure medium supply for the servovalves by an electrical signal, without release by the comparator.
It has already been proposed that the ports of the control spool, which are connected to the outlets of the electrohydraulic servovalves, be associated in a certain manner with a hydrologic system, which detects the error and, for its part, triggers indicators and/or switches. In this previously used system, there must be a separate port for each outlet to each servovalve, i.e., a quantity of positioning lobes equal to the number of servovalves. This leads to inconvenient dimensions, increased manufacturing effort and increased susceptibility to errors attributable to the control spool.
It is therefore the object of the present invention to provide a method and a device of the type mentioned above which permit simpler construction of the control spool, without having to compromise its functioning, especially the possibility of evaluation in a hydrologic system.
The invention attains this objective by employing a method in which the positioning land of the control spool is acted upon, on the one side, by the one signal group consisting of the combined servovalve output signals of the same sign and, on the opposite side, by the central pressure media supply, while the remaining free servovalve signals are applied individually to the hydrologic system.
The summation of all output signals, always essential in a "majority voting system", no longer takes place, in the case of the proposed device mentioned above, such that the forces produced by the individual pressures and the associated land areas are added, the resultant force depending upon the difference between the output signals of opposite sign. Instead, in the case of the present invention, the signal group consisting of all output signals of the same sign is combined and their combined pressure is compared with the constant pressure in the central pressure media supply. The free remaining output signals of the opposite sign therefore are left over for a further processing in the hydrologic system. The invention therefore exploits the fact that both output signals of a servovalve always change with respect to one another, so that a qualitative and quantitative indication as to the condition of the servovalve can also be obtained by comparing one output signal to a constant valve.
To realize the method outlined in the invention, a device is provided having a multiplicity of electrohydraulic servovalves, a control spool triggered by the servovalves to control a hydraulic actuator and a hydrologic system wherein the one group of all servovalve outputs of the same sign is conducted to one side of the positioning land of the control spool, and the other group of outputs is connected separately to the hydrologic system and, further, wherein, the other side of the control spool positioning land is connected with the central pressure medium source.
One illustrative embodiment of the invention provides, as servovalves, two single stage jet-pipe valves attached to a central pressure medium source, further, a control spool having one positioning land, two control lands and an attached comparator responding from two sides, said comparator having a switching device to shut off the central pressure medium supply, each of the two jet-pipe valve outputs of the same sign jointly acting upon one side of the positioning land, the central pressure medium supply acting upon the other side of the positioning land, and the two other outputs of the jet-pipe valves acting upon both sides of the comparator, the displacement of which triggers the switching device.
The positioning land mentioned in the present description can readily be divided into two individual lands, in which case, only one side of each land is acted upon for the purpose of displacing the control spool.
Another illustrative embodiment of the invention provides that two such devices be connected in parallel to a common pressure medium supply, in which case, both control spools are rigidly, mechanically connected to each other and the output signals trigger the switching devices indirectly via an AND-gate and a further switch for cutting in a safety device or to cut out the central pressure medium supply. A circuit of this kind still permits control of the control valve even after the shutoff of two servovalves by one of the switching devices.
In still another illustrative embodiment of the invention, three electrohydraulic servovalves are provided whose two free outputs not used to trigger the common control spool, trigger a system of three hydraulic comparators whose outputs are connected via a triple AND-gate to a switch for cutting off the central pressure medium supply or to any other safety circuit. A circuit of this kind can survive the failure of one servovalve. Not until failure of a further servovalve do all three comparators deflect and activate, via the AND-gate, the switch for any safety circuit.